Several processes are known in the art for the polymerization of HFPO. For example, U.S. Pat. No. 3,660,315 and JP-B 5360/1978 disclose a process for polymerizing HFPO using a tetraglyme solution of a compound of the following formula (1a) as a polymerization initiator, to thereby produce a difunctional polymer of the following formula (2a). ##STR1##
This process for the polymerization of HFPO is characterized by using a homogeneous solution which is obtained by mixing cesium fluoride, tetraglyme, and FOCCF(CF.sub.3)OCF.sub.2 CF.sub.2 OCF(CF.sub.3)COF and removing the excess of cesium fluoride. This prevents the homopolymerization of HFPO catalyzed by the excess of cesium fluoride and eventually suppresses the formation of a monofunctional (that is, one end functional) HFPO polymer.
J. Macromol. Sci.-Chem., 48 (3), 499-520 (1974) describes that the presence of hexafluoropropene (abbreviated as HFP, hereinafter) prevents chain transfer during HFPO polymerization, thereby increasing the degree of polymerization of the resulting polymer. The effect of HFP is allegedly to prevent the chain transfer by trapping free fluoride anions.
It is important that the polymerization of HFPO be carried out under sufficient conditions to prevent the chain transfer, that is, the initiation of polymerization from a chemical species other than the initiator. To this end, the polymerization temperature should be kept as low as possible.
Well known in conjunction with the polymerization of HFPO is the tendency that as the polymerization temperature lowers, reaction selectivity increases so that the degree of polymerization of the resulting polymer increases and the formation of a monofunctional polymer (by-product) is minimized when a difunctional initiator is used.
Now that the product is a polymer, the attempt to lower the temperature, however, encounters the limited range of practically acceptable temperature because the polymerization system tends to be viscous and becomes thickened at lower temperatures. In reactors of the laboratory scale, even the contents of high viscosity can be cooled. In large-size reactors of the commercial scale, which have a relatively small heat transfer area, it is impossible to gain the same cooling efficiency as in the small-size reactors unless special means is devised for increasing the heat transfer area. More particularly, in a system of cooling the wall of a conventional cylindrical reactor, the quantity of HFPO that can be polymerized per unit time is governed by the effective heat transfer area rather than the internal volume of the reactor. Additionally, since the coefficient of heat transfer across the wall drastically drops as the contents increase the viscosity, the internal temperature rises unless the feed rate of HFPO is reduced to decrease the amount of exothermic heat (polymerization heat). On scale-up manufacture, this apparently causes serious drawbacks including an increased feed time of HFPO and a decline in product quality due to an increased polymerization temperature.
There is a desire to have a means for lowering the viscosity of a HFPO polymerization system.